The present invention relates to a connector for anchoring a first building structural member to a second building structural member. The connector works in conjunction with a separate anchor member that is received or attached to the second building structural member and with fasteners for attaching the connector to the first building structural member.
Earthquakes, hurricanes, tornadoes and floods all impose forces on a building that can cause structural failure. To counteract these forces, it has become common practice to add or strengthen ties between the structural members of a building in the area of the building where these cataclysmic forces may be concentrated. For example, framed walls can be connected to the foundation rather than merely rest on it. Connections between the framed walls of each floor can be strengthened. And joists can be connected to both their headers and the walls that support the headers. One of the most common connectors designed for these applications is commonly called a holdown. Holdowns are commonly used to anchor framed walls to the foundation. Holdowns restrain wall posts or studs against uplift, particularly uplift at the ends of shear walls that occurs as a result of lateral loads being applied to the top of the shear wall. When lateral loads, such as may be caused by earthquakes and high wind, are applied to the top of a shear wall, the shear wall tends to overturn rather than collapse, because it is reinforced against lateral shear loads. The overturning moment tends to lift the trailing side of the shear wall.
Early holdowns were constructed from two or more separate pieces of metal that were welded together. Welded holdowns had to be painted to prevent rust. They were heavy and costly to produce, in particular because of the additional labor involved in welding and painting.
State of the art holdowns are made from galvanized sheet metal formed on progressive die machines. Recently, strong and light cast materials such as aluminum have also been used. Ideally, state of the art holdowns require no welding or painting. These advances have reduced the cost of making holdowns while increasing their ability to withstand tension forces. Severe earthquakes in California and Japan demonstrated that holdowns that are capable of being mass-produced and installed inexpensively should be made even stronger for many connections.
Typical holdowns work in conjunction with a separate anchor member and are attached to the side face of the first building structural member, which is generally a vertically-disposed wall stud. In these holdowns that attach to the side of a stud or post, the anchor member is attached at the seat of the connector. The seat is connected to a back member and the back member attaches to the side face of the stud or post. Often, these holdowns have one or more side members to increase the strength of the connector or to connect the seat member to the back member.
Another style of holdown attaches to the bottom end of the stud or post. A patented example of this type of holdown is found in U.S. Pat. No. 6,513,290, granted to William F. Leek on Feb. 4, 2003. The advantage of a holdown that attaches to the bottom end of a post or stud is that it can remove any eccentricity from the connection. Eccentric connections introduce bending stresses into the post or stud. Thus, as a shear wall moves back and forth under shear loads, the post or stud attempts to rotate about its base. Prior art holdowns, as described above, do not allow post rotation at the bottom, and bending stresses are introduced into the post above the holdown during shear wall movement. Concentric holdowns tend to allow a degree of lateral rotation because the holdown and the supported post are in line with the anchor bolt and the axis of rotation is generally on that line. The point at which the holdown is restrained to the anchor bolt is where rotation will tend to occur, if rotation is possible. Prior art holdowns, however, have not allowed rotation to occur at that point, but have instead permitted bending in the wood post at the juncture of the top of the holdown and the supported post. Eccentric holdowns tend to resist rotation in one direction only because they are attached to one side of the post and therefore to one side of the natural axis of rotation. In the present invention, the axis of rotation is below the post or stud and immediately below the point at which the holdown is restrained to the anchor bolt.
The present invention improves on the prior art by lowering the axis of rotation below the top of the anchor member that connects the holdown to the second structural member, or foundation. It is advantageous to have the axis of rotation as low as possible. Under gravity loads, the post pushes down on the concentric holdown below it along a load line of action. When the post is perfectly vertical, the load line of action passes through the middle of the holdown. When the post is rotated and compressed, as under racking deformation during an earthquake, the load line of action follows the line of the post and the projection point at which it intersects the underlying structural member moves away from the center of the holdown base. The higher the axis of rotation, the further the line of action moves for any given degree of rotation. If the line of action moves beyond the holdown base, the holdown base will tend to be pushed over because the top of the holdown base will be pushed toward a point beyond the bottom of the holdown base. Although the holdown base will tend to be pushed over, it is restrained, primarily by the anchor bolt. As the load line of action moves away from the center of the holdown, the compression stresses on the underlying structural member become increasingly non-uniform. This is undesirable because it tends to rupture the progressively bending upright post member and to damage the underlying structural member, usually a concrete foundation, which both supports and anchors the structure above it. The present invention lowers the axis of rotation so that the load line of action passes through the holdown base substantially away from its edges, keeping the bearing stresses on the underlying structural member as uniform as possible.
Recently, it has become apparent that simply increasing the strength of holdowns does not necessarily result in the best connection for the most common installation, between a frame wall reinforced for shear resistance and a concrete foundation. The greater the simple strength of the holdown, the more rigid its connection. It is possible to design and manufacture holdowns that are so strong and rigid that failure is bound to occur elsewhere in the connected structure, such as in the load bearing wood member above the holdown. By transferring failure entirely out of the holdowns, the risk of catastrophic failure of the connected structure is increased. It has therefore become desirable to design holdowns that maximize resistance to uplift forces, but which allow a small amount of rotation to occur at the connection to the second structural member or foundation.
The present invention improves on the prior art by allowing for rotation as close as possible to the point of attachment to the underlying structure. This means that there will be very little bending in the post or stud to which the holdown of the present invention is attached. This allows the post or stud to be smaller and of a lower stress grade. Because the post or stud bends very little, the attachment of the post or stud to a shear-resisting member, particularly to a panel by many small fasteners such as nails, works better, distributing and dissipating shear forces more evenly and effectively throughout the panel. This results in more gradual and predicable failure. The holdown of the present invention provides a hinge joint at the base of the holdown that improves on prior art rigid holdowns that raised the axis of rotation and tended to transmit additional tension forces into the anchor bolt when the shear panels levered up on their lateral corners. Without the hinge of the present invention, the tension forces acting on the anchor bolt and the prior art holdown are greater than the uplift force of the post or stud alone when the post is subject to an overturning moment. When prior art eccentric or concentric rigid holdowns resist post or stud rotation, additional tension forces are created in the anchor bolt.
The most preferred form of the present invention is superior to prior art rotating concentric holdowns because the present invention does not rely on a relatively weak horizontal pin connection for both rotation and the transfer of both post uplift forces (by shear) and compression forces (due to gravity or overturning moment) between vertical plates embedded in the post or stud and a base that is connected to the anchor bolt. The present invention transfers compression forces by direct bearing of one contact surface on another, as in the most preferred embodiment in which a standoff base and channel slide between the bottom surface of a floating washer and the upper surface of a support base, from post to standoff base to strap to support base to foundation. The present invention transfers uplift forces directly to the anchor bolt in tension (aside from the post to strap connection). In its most preferred embodiment, the present invention achieves rotation with sliding surfaces rather than a pin connection that, if damaged or bent, could lead to failure of the gravity load system or rotation system.
The holdown connector of the present invention improves on the prior art by providing a holdown that withstands very high tension loads with minimal deflection, while allowing for rotation about an axis lower than the top of the anchor member, and being economical to produce.